This invention relates to a flue gas treating process for removing at least dust and sulfur dioxide from the flue gas of a coal-fired boiler or the like by use of a dry electrostatic precipitator and a desulfurizer based on the wet lime-gypsum method. In particular, it relates to a flue gas treating technique which makes it possible to reduce the size of the dry electrostatic precipitator markedly and achieve a considerable saving of cost.
An example of a known high-performance flue gas treating system for removing dust (such as fly ash) and sulfur oxides (chiefly sulfur dioxide) efficiently from flue gas discharged from a coal-fired boiler or the like is disclosed in Japanese Patent Publication No. 56377/'95.
According to this process, as illustrated in FIG. 1 of the aforementioned patent, flue gas from a coal-fired boiler is cooled to a temperature of 80 to 110.degree. C. by means of an air heater (AH) or the heat recovery section of a gas-gas heater (GGH), introduced into a dry electrostatic precipitator (dry EP) in order to reduce the dust concentration of the flue gas to 100 mg/Nm.sup.3 or less, and then introduced into an absorption tower of a combined type desulfurizer based on the wet lime-gypsum method. In this absorption tower, the flue gas is brought into contact with a slurry containing a calcium compound as the absorbent, whereby the sulfur oxide concentration is reduced and, at the same time, the dust concentration thereof is reduced to a final target value of 10 mg/Nm.sup.3 or less.
That is, in this process, the temperature of the flue gas introduced into the electrostatic precipitator is made lower than in the prior art (120-160.degree. C.) to reduce the specific resistance of dust. Thus, a back discharge phenomenon in the electrostatic precipitator can be avoided to reduce the size of the electrostatic precipitator and improve the performance thereof. Moreover, the dust present on the outlet side of the electrostatic precipitator consists essentially of rescattered dust particles which have aggregated and enlarged, so that the dedusting capability of the desulfurizer is also improved.
Furthermore, heat recovery by a gas-gas heater, which has hitherto been carried out on the downstream side of the electrostatic precipitator, is carried out on the upstream side of the electrostatic precipitator. As a result, a large amount of dust is present in the flue gas undergoing heat recovery by the gas-gas heater, so that sulfur trioxide present in the flue gas can be prevented from condensing in the heat recovery section of the gas-gas heater and producing a harmful mist. This also makes it possible to enhance the degree of dust removal in the electrostatic precipitator without consideration for such problems with sulfur trioxide.
Accordingly, the above-described flue gas treating process is excellent in that, when compared with the constructions of the prior art (e.g., the construction shown in FIG. 4 of the aforementioned patent), various requirements (i.e., an improvement in performance and a reduction in the size and cost of equipment) which are becoming severer year by year in this technical field from the viewpoint of global environmental protection can be satisfied to a considerable extent.
However, in the flue gas treating process a disclosed in the aforementioned patent, the dust concentration of the flue gas leaving the dry electrostatic precipitator and introduced into the absorption tower is adjusted to 100 mg/Nm.sup.3 or less (usually about 30-50 mg/Nm.sup.3). Thus, most of the dust present in flue gas at a concentration of about 10,000 to 20,000 mg/Nm.sup.3 is captured and removed in the electrostatic precipitator, and a slight amount of residual dust is removed in the absorption tower of the desulfurizer. Consequently, although the size of the dry electrostatic precipitator can be reduced as compared with the prior art, the electrostatic precipitator still has a large size and involves a considerable cost. Thus, it is desired to achieve a further reduction in size and cost.
The conventional conception which requires the dust concentration to be reduced to 100 mg/Nm.sup.3 or less in the electrostatic precipitator is based on the conventional technical common sense that sufficiently high dedusting capability cannot be achieved in the absorption tower of the desulfurizer and, therefore, most dust must removed in the electrostatic precipitator so as to reduce the final dust concentration to 10 mg/Nm.sup.3 or less. However, such sharing of the dedusting load has not necessarily been optimal.
Furthermore, even if sufficiently high dedusting capability can be achieved in the desulfurizer, the combined type desulfurizer still involves problems such as a reduction in the purity of gypsum obtained as a by-product due to contamination with dust, and a loss in the activity of limestone used as the absorbent. With consideration for these problems, the dedusting load of the electrostatic precipitator has been extremely high in the prior art.